Which algorithm is primarily used to establish a shared secret over an untrusted network rather than directly encrypting large messages?

• A. Diffie-Hellman
• B. RSA
• C. AES
• D. ElGamal

Answer: (A)

The essential idea here is a method for two parties to agree on a secret key over an insecure channel, so they can then use that key to encrypt large messages with a fast symmetric cipher. Diffie-Hellman is specifically designed for this purpose: each side contributes a private piece and a public value, and by combining them they end up with a shared secret that an eavesdropper cannot derive without solving a hard math problem. This shared secret becomes the session key for subsequent encryption, which is why Diffie-Hellman is described as a key-exchange protocol rather than a method for encrypting data directly.

RSA, AES, and ElGamal serve different primary roles. RSA is used mainly for public-key encryption and digital signatures, not for establishing a session key in the standard sense. AES is a fast symmetric cipher used to encrypt actual data once a key is known. ElGamal is an asymmetric encryption scheme, not a dedicated key-exchange primitive, though it can be used in various encryption schemes. Diffie-Hellman uniquely targets the task of creating a shared secret over an untrusted network, which is then used to protect large messages.